1. Field of the Invention
The present invention relates to zener diodes and more particularly to zener diodes adapted for mounting in a double plug package.
2. Description of the Prior Art
In semiconductor devices especially discrete devices such as diodes and transistors, a substantial portion of the cost of the device is attributable to packaging. Therefore, in the semiconductor art, there is a need for inexpensive packages that may be assembled in a minimum amount of time and preferably with automated assembly equipment.
The double plug package has been particularly useful for packaging diodes since it is inexpensive and subject to automated assembly while providing a hermetic seal; however, difficulty has been experienced with zener diodes in double plug packages.
Heretofore, one contact element of a zener diode was positioned directly over the shallow junction region which resulted in an unacceptable number of rejected devices resulting from shorted junctions. When the metallic contact was placed directly over the shallow junction region, alloying of the metal contact into the semiconductive material would take place during the high temperature sealing operation required of double plug packages. Said alloying caused contamination of the shallow junction region which resulted in high leakage and shorts.
It has also been discovered that alloying was not the only problem resulting from the formation of a contact directly to the shallow junction region. The manufacturing yield for high voltage zener diodes was also poor and on such devices the junction depth is sufficient so that alloying does not create a short. It was determined that surface imperfections between the contact and the silicon material caused the contact to make lower resistance contact to the silicon at certain points and as a result, high current density or current crowding would occur at these points causing avalanching and premature breakdown of the junction. It was also discovered that the pressure exerted on the silicon material during packaging of the zener diode in the double plug package had an adverse effect on the shallow junction, resulting in poor yields and unpredictable diode characteristics.
Thus, the double plug package which required sealing at a temperature of at least 680.degree. C. was not adaptable for use with low voltage zener diodes and also experienced poor yields when used for packaging higher voltage zener diodes.